The present invention relates to radar systems, and more particularly to a monopulse radar system for determining the height of a cooperative or non cooperative detected target.
A majority of the air traffic control (ATC) type radar systems that are currently in operation are relatively low cost two dimensional (2D) radars. Radars of this class include fixed-site civilian airport ATC radars as well as mobile military ATC radars. In these radars, a specially shaped reflector antenna, in conjunction with a pair of feeds, forms a pair of squinted fan beams that provide broad elevation detection coverage. The two feeds generate a pair of squinted beams called main and auxiliary. The elevation coverage is typically either 0 to 30 degrees or 0 to 40 degrees depending on the particular radar system. To optimize energy distribution, the fan beams have a cosecant-squared beam shape that provides long range coverage at low elevation angles and constant altitude coverage at all ranges. Traditionally, these systems are referred to as 2D because they only measure a target""s range and azimuth.
To obtain elevational information on the targets, typical ATC radars are equipped with a secondary surveillance radar (SSR), from which a cooperating target""s elevation information is obtained. The SSR sends out an interrogation signal and then receives a reply from the transponder located on the aircraft. The aircraft""s transponder replies with a message that contains aircraft height above sea level information based on its own barometric sensor. Most civilian, commercial and military aircraft are equipped with transponders. But, for non-cooperating targets, that have defective transponders or deliberately operate with their transponders turned off, ATC radars cannot rely on the SSR to obtain the target""s elevation information.
In order to obtain elevation information of a non-cooperating target using ATC type radars, receive beam data processing called monopulse radar angle estimation has been utilized. The main feed transmits the radar energy, and the target echo is received at each of the two feeds. On receive, the main feed, in conjunction with the reflector, generates the main receive fan beam. The auxiliary feed, positioned below the main feed, in conjunction with the reflector, generates a second receive fan beam that is squinted a few degrees higher in elevation from the main receive fan beam. Since it is higher in elevation, the auxiliary beam is useful for mitigation of near-in clutter. Each feed is connected to receive channel processing components, which includes a low noise amplifier, a down converter, a pulse compressor, and a Doppler filter bank. The output of the Doppler filter consists of complex data, or otherwise known as inphase (I) and quadrature (Q) data (hereinafter collectively referred to as the xe2x80x9cI/Qxe2x80x9d data). Monopulse processing is performed on the receive beam data and uses the I/Q data from the two receive channels to estimate target""s elevation angle which is used to calculate the height of the target.
The monopulse radar angle estimation methods in use until now have relied on real-valued lookup tables. For example, in squinted-beam ATC radar systems, the prior art monopulse radar angle estimation methods used either the amplitude data of the receive beams or the phase data of the receive beams, but not both. The prior art amplitude-monopulse method extracts the magnitude of the I/Q data in each receive channel at the Doppler filter output, computes a ratio of the amplitudes, which is a real number, and then uses a real-valued lookup table to convert the ratio to the target""s elevation angle. The real-valued lookup table contains a set of reference amplitude ratios that are correlated to a set of elevation angles. The prior art phase-monopulse method extracts the phase of the Doppler filter I/Q data in each channel, computes the phase difference between the channels, which is a real number, and then uses a real-valued lookup table to convert the phase difference to the target""s elevation angle. The real-valued lookup table in this case contains a set of reference phase differences that are correlated to a set of elevation angles.
The aforementioned prior art squinted-beam methods for the ATC type radars are limited in their performance. The prior art amplitude-monopulse method provides valid estimates of target""s elevation angle over a small range, only from 0 to about 4 degrees (within the radar""s 0 to 30 degree detection coverage), which is much narrower than desired. The prior art phase-monopulse method provides a wider coverage, but is susceptible to a phase wrap-around problem induced by noise or interference. For example, a target at 28-degrees elevation might be estimated as being at 2-degrees elevation. Thus, an improved monopulse radar elevation angle estimation method for ATC type radars is desired.
According to an aspect of the present invention, there is disclosed a new monopulse radar angle estimation process, hereinafter referred to as the general complex (GC) monopulse method, and its application in radar systems for determining the elevation of a detected non-cooperating target. Application of the GC monopulse method allows the dual beam radar to determine the elevation angle of the target, and in turn the height of the target, more accurately. Although xe2x80x9cmonopulsexe2x80x9d means one pulse, in the context of the present invention, the term is used in reference to receive processing of the radar echo signal and xe2x80x9cmonopulsexe2x80x9d means one coherent burst of pulses.
Monopulse radar angle estimation requires data to be received simultaneously in two receive channels. Data in each channel, at the Doppler filter output, consists of a complex number representing the complex envelope of the signal, plus noise, that was pulse compressed and Doppler filtered. This complex number consists of a real component and an imaginary component, commonly known as the inphase/quadrature (I/Q) data. This complex data from one channel is divided by the complex data in the other channel to form a complex ratio. The GC monopulse method uses a complex lookup table to convert the complex ratio to the target elevation angle which is then used to calculate the target height. The complex ratio of the two beams (receive channels), is compared against a set of reference complex ratios in the complex lookup table to determine the corresponding elevation angle.
Although, the GC monopulse method works for arbitrary beam patterns, it is particularly advantageous in squinted beam radar systems. The GC monopulse method provides better accuracy, supports a wider range of operation, and has better wrap-around rejection compared to the prior art amplitude-only or the phase-only monopulse methods. Two examples of practical beam patterns are: a pair of squinted pencil-beams, or a pair of squinted cosecant-squared fan-beams. A pair of squinted pencil-beams can be formed either by a parabolic shape reflector with two vertically separated feeds, or by a phased-array radar with two beam formers and two electronically steered full-aperture sum-beams. Similarly, a pair of squinted cosecant-squared receive fan-beams can be formed by a specially shaped reflector with two vertically separated feeds, as is the case with the ATC type radar system described herein. The two receive beams in the ATC type radars, are referred to as elevation fan beams, substantially cosecant-squared in shape, and are squinted in elevation.
Thus, disclosed herein according to an embodiment of the present invention is a radar system for determining the height of a target. The system comprises a transmitter for generating a pulse of radio energy; an antenna for emitting the pulse from the transmitter and receiving a target echo signal. The system further comprises two receive channels electrically connected to the antenna for receiving two receive beams: a main receive beam from the main feed and an auxiliary receive beam from the auxiliary feed. The system receives I/Q data from each of the two receive beams, and calculates a compensated complex-ratio of the two I/Q data (hereinafter referred to as the xe2x80x9ccompensated received complex-ratioxe2x80x9d). Finally, a conversion unit converts the compensated received complex-ratio to the target""s height.